A syringe has been used since the early part of the twentieth century to inject air into the vitreous cavity of an eye to correct a retinal detachment of the eye. In a procedure known as a "pneumatic retinopexy," the injected air gives tamponade to a retinal hole in the eye and holds the retina of the eye in a position of reattachment until the adhesive modality between the retina and the pigment epithelium of the eye has achieved adequate strength. Such injection of air as a medical procedure is referred to as "insufflation," and for the case of injection into the vitreous cavity, "intravitreal insufflation." The injection of air into the eye may be performed for other medical procedures as well. For example, when the vitreous of an eye is surgically removed, during vitrectomy, the air may be used to provide full tamponade following such procedure, also for purposes of reattaching the retina. Some surgeons have used air for total ocular fluid replacement even without retinal detachment, with good results and apparently no recognized complications.
The use of air for intravitreal insufflation in a pneumatic retinopexy or after vitrectomy has the advantage of no known toxicity. However, a major drawback associated with the use of air is that it may be absorbed by body tissue more rapidly than desired, that is, before a firm pigment epithelium-retinal adhesion has formed, thereby limiting its usefulness as an effective tamponade. However, despite its lack of chemical toxicity, air may have the additional disadvantage of causing physical problems within the eye. For example, if a patient is being operated under general anesthesia, nitrous oxide should be discontinued ten minutes prior to the air insufflation to allow clearance of the nitrous oxide from the body. The discontinuance of nitrous oxide is required because it is a soluble gas and rapidly moves from the venous blood into the injected intravitreal bubble of gas, thereby causing rapid expansion of the bubble with a simultaneous rise in intraocular pressure. However, the nitrous oxide leaves the bubble just as rapidly, so that if it has not been discontinued, the intravitreal bubble rapidly shrinks when the anesthesia is discontinued, giving a lesser degree of insufflation than desired.
To alleviate the problem of air absorption into the body, a fluid having a slower rate of tissue absorption is desirable. Gases which may be used as an effective substitute for air in ophthalmic procedures are sulfur hexafluoride (SF.sub.6) or perfluoropropane (C.sub.3 F.sub.8). Sulfur hexafluoride is a chemically inert gas which has not proved to be toxic, is highly lipid soluble, and has a very low water solubility. For these reasons, sulfur hexafluoride remains in body tissue pockets for about twice as long as air. As may be appreciated, the duration a sulfur hexafluoride bubble remains within the intraocular region of the eye depends on the variables of the initial concentration and the volume injected. The greater the amount of the either of the variables, the more the bubble of sulfur hexafluoride expands, thereby maintaining its presence as a therapeutic tamponade over a longer period of time. However, when pure sulfur mexafluoride is used in a tissue pocket, such as a vitreous cavity, its volume expands for about two days, which causes substantial increases in intraocular pressure. Therefore, the vitreous cavity should never be completely filled with pure sulfur hexafluoride.
Intraocular pressure elevation resulting from sulfur hexafluoride volume expansion is usually avoided if the vitreous cavity is not filled with more than 40% pure sulfur hexafluoride. An alternative to such a procedure is to inject a precisely measured mixture of sulfur hexafluoride and air in such a ratio which corresponds to the ratio that would develop spontaneously in the eye if the vitreous cavity were filled with pure sulfur hexafluoride to not more than 40% of the volume of the eye, as in the aforementioned procedure. In this regard, it has been discovered that for clinical purposes, a mixture composed of 40% sulfur hexafluoride and 60% air will not expand. Accordingly, it has been found that such a fluid mixture can be used to fill the entire vitreous cavity during an ophthalmic procedure without resulting in a substantial increase in intraocular pressure. Therefore, there exists a need for an apparatus to efficiently and accurately prepare a fluid mixture, such as a mixture consisting of 40% sulfur hexafluoride and 60% air, for injection into body tissue, such as an eye, during a medical procedure such as a pneumatic retinopexy.
While widely diverse in construction and operation, prior art apparatuses and methods for preparing a fluid mixture in a syringe for injection into body tissue, such as an eye, are replete with deficiencies and shortcomings which have detracted from their usefulness. Foremost among the deficiencies of the previously employed apparatuses is their apparent inability to accurately and efficiently prepare a fluid mixture in a syringe for injection into the intraocular region of the eye. Previously, to prepare a fluid mixture consisting of 40% sulfur hexafluoride and 60% air, a syringe was first connected to a pressurized lab cylinder filled with air and the syringe was filled to a predetermined volume. The syringe was then disconnected and connected to a pressurized lab cylinder filled with sulfur hexafluoride, and the syringe was filled to a predetermined volume. As may be appreciated, such a procedure is awkward and time consuming, and requires a number of persons to accomplish.
Another deficiency of the previously employed apparatuses is the apparent unavailability of a commercially produced apparatus which conveniently fills a syringe with either an air or a fluid mixture. Typically, doctors and surgeons have constructed makeshift apparatuses for filling a syringe with air or fluid. Although such apparatuses may have accomplished their purposes with varying degrees of success, they are inaccurate, and in some cases unsafe. More particularly, these makeshift apparatuses may permit the fluid mixture to leak from a syringe during the preparation of the fluid mixture, thereby producing inaccurate measurements which affect intraocular eye pressure. Safety also may be compromised if regulating valves are not employed with such makeshift apparatuses, as it is common for an end of a syringe to blow out, thereby introducing flying debris and jeopardizing the safety of all personnel present.
Still another deficiency of the prior art devices is the inordinate amount of care required to ensure a sterile transfer of air or gas to the syringe. As may be appreciated, to prepare the syringe with a fluid mixture, the syringe is connected to a pressurized air source and then disconnected, and then connected to a pressurized source of gas, such as sulfur hexafluoride, and then disconnected. The sterility of the syringe is thereby placed at risk because of the multiple steps which place the syringe in contact with multiple foreign objects before the bubble is injected into the intraocular region of the eye.
Yet another deficiency attendant with previously employed devices is their relative unpredictability in operation resulting from their makeshift design. Typically, these devices were produced by doctors or surgeons from spare parts which may not have been designed for such use. Accordingly, there is an increased likelihood that these apparatuses will fail or otherwise malfunction during operation.
Still another deficiency of the previously employed devices is their apparent inability to reduce waste of compressed air or compressed gas. As previously described, compressed air and compressed gas typically have leaked from the laboratory cylinders or the syringe during the syringe preparation process. This, of course, wastes resources and causes an increase in attendant medical costs.
Accordingly, a need has arisen for an apparatus which quickly, effectively and accurately prepares a single dose of fluid for injection into body tissue, and which is disposable and convenient, thereby rendering such an apparatus particularly suitable for use in a surgical operating room, or in a medical doctor's office for out-patient medical procedures.